ε‐Ga2O3: An Emerging Wide Bandgap Piezoelectric Semiconductor for Application in Radio Frequency Resonators

Abstract The explosion of mobile data from the internet of things (IoT) is leading to the emergence of 5G technology with dramatic frequency band expansion and efficient band allocations. Along with this, the demand for high‐performance filters for 5G radio frequency (RF) front‐ends keeps growing. The most popular 5G filters are constructed by piezoelectric resonators based on AlN semiconductor. However, AlN possesses a piezoelectric constant d 33 lower than 5 pm V−1 and it becomes necessary to develop novel semiconductors with larger piezoelectric constant. In this work, it is shown that strong piezoelectricity exists in ε‐Ga2O3. High‐quality phase‐pure ε‐Ga2O3 thin films with a relatively low residual stress are prepared. A switching spectroscopy piezoelectric force microscope (SS‐PFM) measurement is carried out and the piezoelectric constant d 33 of ε‐Ga2O3 is determined to be ≈10.8–11.2 pm V−1, which is twice as large as that of AlN. For the first time, surface acoustic wave (SAW) resonators are demonstrated on the ε‐Ga2O3 thin films and different vibration modes resonating in the GHz range are observed. The results suggest that ε‐Ga2O3 is a great material candidate for application in piezoelectric devices, thanks to its wide bandgap, strong piezoelectric property, small acoustic impedance, and low residual stress.


TEM measurement
The 1.2-μm-thick ε-Ga 2 O 3 sample was characterized by TEM in Figure S1. The film is pure ε-phase confirmed by the selected area electron diffraction (SAED) in Figure   S1(b), where the diffractions of (002) and (030)

Measurement of wafer bowing and residual stress by XRD
For wafer bowing measurement, the ω-scan of ε-Ga 2 O 3 (004) plane was first carried out in the center of the wafer, deriving a diffraction peak at ω c . Then the holder moved by L=+/-20 mm along the x-direction to perform ω-scan at the left/right side of the wafer, deriving the diffraction peak at ω L /ω R . The diffraction geometry in Fig. S1(b) shows the measurement at the left side of the wafer. The curvature of the wafer is hence defined by κ≈ | | | | . The wafer bowing height H could be calculated by H≈|L|×tan(△ω/2). As the diffraction geometry shown in Fig. S2, ω L >ω c >ω R means convex wafer bowing and compressive residual stress. On the other hand, the sample is concave bowing and under tensile residual stress when ω L <ω c <ω R . The resolution of ω-scan was 0.004° in this work, corresponding to a resolution of wafer bowing of ~1.4 μm. After the extraction of wafer curvature κ, the residual stress could be calculated according to Stoney's Equation.
The wafer bowing measurement of sample grown on Sapphire substrate with thickness of 1 μm is shown in Fig. 1(c), and the measurement for the other two samples are shown in Fig. S3. It should be noticed that the diffraction intensity has not been normalized. The results of Fig

SS-PFM measurement of ε-Ga 2 O 3
The SS-PFM measurements were carried out to investigate the piezoelectricity of ε-Ga 2 O 3 . During the measurement, a DC voltage V dc scanning between -V max~+ V max was applied on the sample. During the scanning, the V dc was turned on and off, and an AC voltage V ac was applied on the sample to excite piezoelectric oscillation of the sample when the V dc was turned off [ Fig. S4 (a)]. The amplitude of V ac was set to be a constant of 0.8 V, while the maximum value of V dc is changeable. Once the V ac was applied on the sample, the sample would oscillate due to piezoelectricity and the phase and amplitude of oscillation was collected. As a comparison, SS-PFM measurement was also carried out on a GaN/AlN (2300/30 nm) heterostructure. The DC voltage scanned through -5 V~+5 V, -7 V~+7 V and -9 V~+9 V, and the results are shown in Fig. S4

Structural properties of the AlScN thin film
The comercial AlScN sample used in this work was grown on Sapphire substrate with a Sc composition of ~40%. The thickness of the film is 1.2 μm. The XRD 2θ-scan in Figure S6(a) presents a diffraction peak at 35.87°, which belongs to the (002) plane of AlScN. The diffraction peak at 41.85° corresponds to Sapphire(006) plane. The HRXRD rocking curve of AlScN(002) plane in Figure S6(b) derives a FWHM of 0.04°, which means that the AlScN were also grown with high crystal quality.

Extraction of mass density of ε-Ga 2 O 3 by FEM calculation
The SAW resonators were constructed by IDTs with 80 pairs of periodic fingers, and the width W, spacing S and period λ of fingers are 0.6 μm, 0.6 μm and 2.4 μm, respectively. Since the length of the finger L IDT =192 μm is much longer than the wavelength λ, it is acceptable to carried out FEM calculation in a simplified 2-dimension model [ Fig. S7(a)

S7(b)~(d).
There is no experimental report on the mass density of ε-Ga 2 O 3 . Since the density is a critical parameter determining the resonance frequencies of SAW, we have tried to determine the density of ε-Ga 2 O 3 by comparing the measured and calculated resonance frequencies of the SAW resonator. The FEM calculation was carried out by using COMSOL software. The elastic constants C ij of ε-Ga 2 O 3 used for the calculation are shown in Table S1 (Phys. Rev. B 2016, 93, 115204). The piezoelectric stress constants e ij of ε-Ga 2 O 3 used for the calculation are shown in Table S2 (Mater. Res. Express 2018, 5, 036502). It should be noticed that the position of resonance frequency of IDT is affected by mass density, elastic constants and piezoelectric stress constants. However, the mass density and elastic constants play much more important roles since they determine the sound velocity directly. Piezoelectric stress constants have little influence on the frequency position. Generally, the velocity of acoustic wave is determined by the mechanical parameters with the form of v acoustic ∝(C ij /ρ) 1/2 . Since the elastic constants are fixed as shown in Table   S1, the density ρ becomes the only variable during the FEM calculation. The dependence of resonance frequencies on density for the 1.2-μm-thick sample is shown in Fig. S8(a).
The acoustic wave velocities, as well as the resonance frequencies, decrease as the density increases. We define a deviation value to fit the calculated frequencies with the experimental values by:

1/
The subscript i represents different vibration modes. N is the number of observed modes and N=3 for the 1.2-μm-thick sample. The subscript exp and FEM means resonance frequencies derived from the measurement or FEM calculation. As is shown in Fig.   S8(b), as the density increases, the deviation decreases at first and then increases, deriving a minimum value of 2.4% at ρ=4.8 gcm -3 .

Calculation of modulus and sound velocities
The mechanical properties of orthorhombic ε-Ga 2 O 3 was calculated by Voigt-Reuss- Since the density of ε-Ga 2 O 3 has already been determined to be ρ=4.8 gcm -3 , the calculated results are v l =7.6 kms -1 and v s =3.8 kms -1 .